Secondary batteries require their miniaturization and weight-lightening so as to meet the miniaturization and high performances of various devices using the batteries. Further, the a stability, high rate characteristics and good cycle characteristics of the secondary batteries at a high temperature and a high voltage become important in order to be used in electric vehicles. Therefore, various cathode active materials have been studied in order to embody a high-voltage secondary batteries for a such use.
One of cathode active materials is LiCoO2 being commercially available but it has disadvantages in terms of high price and an actual electrical capacity of 140 to 150 mAh/g that is merely 50% of its theoretical capacity. Accordingly, many researches have been actively made to provide another cathode active material for replacing LiCoO2.
Also, Li2MoO3 having a layered-structure or LiMxMn2-xO4 (0<x<2, M is Ni and the like) having a spinel structure has the advantage of high capacity as a cathode active material. However, a lithium salt or an organic solvent in an electrolyte solution decomposes during charging and discharging processes at a high potential (4.9 V) and a high temperature and manganese or molybdenum is released by HF which is produced by a reaction of the lithium salt and moisture. Particularly, these materials may deteriorate charging and discharging characteristics under the condition of a high temperature.
Meanwhile, currently available secondary batteries comprise separator so as to prevent a short circuit between a cathode and an anode and a porous layer made of polyolefin-based resins has been widely used. However, because the polyolefin-based resin has a melting point of 200° C. or less, it is apt to shrink at a high temperature when it was subject to a stretch process for the adjust of pore size or porosity thereof. As a result, when the temperature of a battery rises by internal and/or external factors, the separator may easily shrink or melt, thereby causing a short circuit between a cathode and an anode and resulting in accidents such as battery explosion by the emission of electric energy.
As a separator has a thinner thickness, the discharge capacity of an electrode can have a much more discharge capacity. It is considered because the concentration of a liquid electrolyte around the separator is high and the movement of substances can be facilitated. However, in the case that a polyethylene-based separator is used together with a high-voltage cathode active material, metallic ions released from the cathode active material form a dendrite on an anode and obstruct the pores of the separator, thereby bringing about a rapid decrease in a cycle capacity of the secondary battery at a high temperature.